Understanding fen hydrology - a hierarchical, multi-scale groundwater modeling approach

The hydrologic system that sustains groundwater-fed fens is not very well understood. Fens support a large variety of biodiversity and generally occur along the peripheries of regional groundwater mounds, which are characterized by geo-morphologic variability at multiple spatial scales (from 10s of meters to kilometers). Traditional approaches for understanding fen hydrology have simplified the hydrologic system that supports the fens, such as using one scale of model to represent multi-scale variability in topography and geology. I argue that fens are a product of complexity in topography and geology, and therefore, simplified models cannot be used to completely characterize such systems. In this research I use hierarchical, multi-scale groundwater modeling at two fen sites in southern Michigan to understand the hydrologic processes and mechanisms that sustain these unique ecosystems. The methodology adopted in this research takes advantage of the detailed and extensive hydrologic data that is made available by the GIS and IT revolution in recent decades. The first application of this approach is for modeling the hydrology of the MacCready Fen in Southern Michigan. I built a hierarchy of nested steady-state models to capture the groundwater flow system at spatial scales ranging from the regional ground-watershed to the fen site. A Transition Probability-based approach was used to create a fully 3-dimensional geologic model at the regional and local scales to represent the complex geologic variability. Three-dimensional particle tracking was used to predict the sources of water to the fens and the corresponding delivery mechanisms. The second application is for the geographically-isolated Ives Road Fen in south-eastern Michigan. Although the overall methodology adopted was similar to the one used for MacCready Fen, this site was characterized by very complex 3-dimensional geologic variability that had to be accurately characterized. A detailed 3-D geologic model was created using a Transition Probability approach, which was then incorporated into the groundwater flow model. Results from the multi-scale simulations illustrate the complex and inter-connected nature of the hydrologic system that supports these fens. The water in MacCready Fen can be traced back to a network of sources, including a wetland and a local recharge mound, and Skiff Lake, which is connected to the regional Hillsdale groundwater mound through a "cascade delivery mechanism." A simplified regional scale model or a local scale model would be unable to replicate the complex topography and the 3-D geology that delivers water to the fen. In the case of the Ives Road Fen, in addition to getting water from a local recharge area and a small pond near the fen, it also gets water from the Hillsdale mound and a till plain which moves through a "pipeline," consisting of a confined aquifer beneath a thick clay layer, to deliver water to the fen through a break in the clay layer. Thus, a geographically-isolated fen far away from other fens is hydrologically connected to the same regional mound that provides water to a cluster of fens. The regional mound thus acts as a "master recharge area," since it is the ultimate source of water not only to fens, but also for many rivers, lakes, wetlands and aquifers. The implication of these findings is that rather than protecting individual fens and their immediate surroundings, fens must be managed as part of a much larger, inter-connected groundwater system. The current approach for managing fens and other ecosystems needs to be reassessed and should move away from localized, short-term fixes to system-based, long term solutions.